Method and apparatus for controlling concentration of electrolytic solution

ABSTRACT

In the electrolytic treatment, an acid concentration meter measures the concentration of hydrochloric acid. The difference from a measured value PV b  to a preset value SV b  of the concentration of hydrychloric acid is larger than preset value e, hydrochloric acid is fed out from an acid feeding section. The aluminum concentration meter measures the concentration of the aluminum chloride. A water feed cycle is calculated from a measured value PV a  and a preset value SV a  of the concentration of hydrochloric acid, and a total value of the electrolytic current I generated by a power source section. At every water feed cycle, a water feeding section feeds water to add to the electrolytic solution.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a method and an apparatus forcontrolling a concentration of electrolytic solution which is used forelectrolytic treatment of a metallic material.

[0003] 2. Description Related to the Prior Art

[0004] In general, an aluminum substrate is used as a substrate of apresensitized plate (hereinafter PS plate) for offset printing. When theprinting is made on the PS plate, the hydrophilic property and theink-philic property (or lipophilic property) are controlled.Accordingly, in order to print images of high quality, a surface of thealuminum substrate is roughened to make the surface uniform. As themethod of roughening the surface of the aluminum substrate, there is anelectrolytic treatment. In a production line of PS plate is used anelectrolytic treatment apparatus in which the aluminum substrate isdrenched in the electrolytic solution and generates an electrolyticcurrent is supplied between electrodes in the electrolytic solution tomake the electrolytic treatment.

[0005] Generally, the electrolytic solution used in the electrolyticsolution is a solution in which many components are dissolved, andcontains metallic ions, and one of hydrochloric acid, nitric acid andsulfuric acid or a mixture of at least two of these acids. Accordingly,in order to keep the situation of the reaction of electrolytic treatmentuniform, the concentration of each component in the electrolyticsolution is measured in performance of the electrolytic treatment, andthe data obtained in the measurement is sent to a controller forcontrolling the concentration of the each component in the electrolyticsolution. As a known method of measuring the concentration of eachcomponent, there is a neutralization titration. However, it takes a longtime to perform the neutralization titration, and metal hydroxidesprecipitate in the titration pollutes a cell and a pipe, and are hardlyremoved from them.

[0006] Japanese Patent Laid-Open Publication No.4-19559 solves theseproblems in the method of the neutralization titration. In the method ofthis publication, a mixture solution whose concentration ratio is knownis prepared. Then several physical quantities (temperature, electricconductivity, propagation speed of ultrasonic wave) of the mixturesolution are measured. On the basis of the result of the measurement, adata map of each component that illustrates a relation of the physicalquantities to the concentration is made. Thereafter, the measurement ofplural physical quantities of the electrolytic solution is made, and inreference with the data map, the concentration of each component isobtained from the measured physical quantities.

[0007] Further, in Japanese Patent Laid-Open Publication No.2001-121837,a data table is previously prepared. Then, at least two physicalquantities are measured, and the concentration of each component in theelectrolytic solution containing the metallic ions is obtained from theobtained value of the physical quantities, in reference with the datatable. Thereafter, on the basis of the obtained values and the like inthe measurement, the concentration of the electrolytic solution iscontrolled in a predetermined range in the method of a feed-back method,a feed-forward method, or a combination of these methods, so as to keepthe treatment conditions uniform.

[0008] When the concentration control is performed in single one orcombination of the feed-back and feed-forward methods, a PID control isgenerally utilized. In the PID control are combined a proportionalcontrol (P), an integral control (I) and a differential control (D).However, a device of the PID control is expensive, which causes to makethe cost for equipment higher. Further, in the device of the PIDcontrol, many extra articles for preservation of the device and a personresponsible for the preservation, who has specialized knowledge, arenecessary. Further, the concentration control can be made only with useof a programmable controller. In this case, however, a complicatedcalculation of amount of components to be supplied for the concentrationcontrol must be made. Further, a programmable controller of high speedand high function. Accordingly, the device therefor is so expensive asthe device for the PID control, and high knowledge and skills arenecessary for preservation of the device.

SUMMARY OF THE INVENTION

[0009] An object of the present invention is to provide a method and anapparatus for controlling a concentration of an electrolytic solutioneasily.

[0010] In order to achieve the object and the other object, in a methodfor controlling a concentration of an electrolytic solution used for anelectrolytic treatment of the metallic material, a measured acidconcentration of acid in the electrolytic solution is measured. In theelectrolytic treatment, part of the metallic material is ionized togenerate a salt, and a measured salt concentration is measured. Then atleast one of a predetermined amount of a diluting liquid and a freshacid is added in accordance with the measured acid concentration, themeasured salt concentration and total current of the electrolyticcurrent used for the electrolytic treatment.

[0011] An apparatus of the present invention for controlling aconcentration of an electrolytic solution used for an electrolytictreatment of the metallic material includes an acid concentration meterfor measuring a concentration of acid contained in the electrolyticsolution, and a salt concentration meter for measuring a concentrationof salt which is generated by ionizing part of the metallic material inthe electrolytic treatment. A current meter measures a current value ofcurrent supplied during electrolytic treatment. The apparatus of thepresent invention further includes an acid concentration control meansfor controlling the concentration of the acid, and a salt concentrationcontrol means for controlling the concentration of the salt. The acidconcentration controlling means adds a predetermined amount of a freshacid to the electrolytic solution on the basis of a measured value and apreset value of the concentration of the acid. The salt concentrationcontrol means calculates a cycle of the concentration control of theelectrolytic solution, and add a diluting solution to the electrolyticsolution at every cycle.

[0012] According to the method and the apparatus of the presentinvention, the concentration control of the electrolytic solution can bemade with used of a cheap programmable controller instead of theexpensive controller such as the PID controller. Further, anelectrolytic treatment apparatus can have a simple structure. Thus acost for producing and maintaining the equipment can be decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The above objects and advantages of the present invention willbecome easily understood by one of ordinary skill in the art when thefollowing detailed description would be read in connection with theaccompanying drawings.

[0014]FIG. 1 is a schematic diagram of production line of a PS plate;

[0015]FIG. 2 is a schematic diagram illustrating an electrolytictreatment apparatus to which a method of controlling a concentration ofelectrolytic solution is applied;

[0016]FIG. 3 is a block diagram illustrating a concentration control ofthe electrolytic treatment apparatus in FIG. 2.

PREFERRED EMBODIMENTS OF THE INVENTION

[0017] In FIG. 1, in a PS plate production line 10, a continuous andthin aluminum web 11 is used as a substrate of a PS plate. The aluminumweb 11 is wound in an aluminum roll 12, which is set to the PS plateproduction line 10. In downstream of the aluminum roll 12, an etchingapparatus 13, an electrolytic treatment apparatus 14, an oxidizationapparatus 15, a first coater 16, a second coater 17, a cutter 18, and anstacker 19 are provided on a transporting path of the aluminum web 11.

[0018] Several surface treatments for the aluminum web 11 unwound fromthe aluminum roll 12 are made in the etching apparatus 13, theelectrolytic treatment apparatus 14 and the oxidization apparatus 15.Then the first and second coaters 16, 17 apply solutions ofphotosensitive compounds onto the surface of the aluminum web 11, andthe solutions are dried to form a photosensitive layer. Thereafter, thealuminum web 11 is cut by the cutter 18 into PS plates 11 a havingpredetermined size, and the PS plate 11 a are accumulates assemiproducts by the stacker 19. The method and the apparatus forcontrolling the concentration of the electrolytic solution of thepresent invention can be mainly applied to the electrolytic treatmentapparatus 14.

[0019] As shown in FIG. 2, in the electrolytic treatment apparatus 14,the aluminum web 11 is drenched to an electrolytic solution 20 whichmainly contents is hydrochloric acid. Simultaneously, a current isgenerated to roughen uniformly the surface of the aluminum web 11. Notethat, in the apparatus for electrolytic treatment, there are severaltypes, for example a flat type and a radial type. The electrolytictreatment apparatus 14 is provided with plural path rollers 25 fortransporting the aluminum web 11 in a transporting direction, and anelectrolytic treatment bath 26 is provided on the transporting path ofthe aluminum web 11 supported by the path rollers 25. Note that the pathroller 25 is formed of, for example, chloroprene rubber, inconsideration with protection of the surface of the aluminum web 11 andthe resistance to hydrochloric acid.

[0020] The electrolytic treatment bath 26 is filled with theelectrolytic solution 20, and the path rollers 25 are disposed so as todrench the aluminum web 11 into the electrolytic solution 20. The pathrollers 25 contact the aluminum web 11 with a predetermined contactpressure to guide the aluminum web 11, and tenses the aluminum web 11 inthe transporting direction (or a lengthwise direction of the aluminumweb 11). Further, the electrolytic treatment bath 26 is provided with atemperature controller (not shown), which reduces the fluctuation of thetemperature of the electrolytic solution 20 to keep the predeterminedcondition for the reaction of electrolytic treatment.

[0021] In the electrolytic solution 20 in the electrolytic treatmentbath 26, two electrodes 28 having plate shapes are arranged in thetransporting direction so as to confront to an upper surface of thedrenched aluminum web 11. Note that the number of the electrodes is notdetermined. However, the current density is usually different betweensides and center of each electrode. However, it is necessary that theelectric current density in the electrolytic solution 20 is uniform inthe transport direction of the aluminum web 11. Accordingly, it ispreferable to provide the plurality of the electrodes along thetransporting path of the aluminum web 11. Further, the electrodes 28 islonger than the aluminum web 11 in widthwise direction such that thecurrent may be generated uniform in the widthwise direction. Theelectrodes 28 are connected through bus bars 29 to a power sourcesection 30.

[0022] In this embodiment, alternating current is used as the electriccurrent generated by the power source section 30 for electrolytictreatment. However, the alternate current and the direct current thatare simultaneously generated may be superposed to use as the currentbetween the electrodes. The shape of the alternative current is notespecially restricted, and the alternative current may be a sinusoidalcurrent, a rectangular current, a triangular wave current and the like.Further, a cycle of the alternative current, a DUTY ratio and the likeare set to adequate values, so as to control a reaction period forreaction of the aluminum web as anode and that for reaction of thealuminum web as cathode. Thus the roughening treatment of the surface ofthe aluminum web 11 is uniformly made.

[0023] The electrolytic treatment apparatus 14 is used in combinationwith a concentration control device constructed of a solution tank 32and a controller section 38. The solution tank 32 is positioned belowthe electrolytic treatment bath 26, and they are combined through afeed-back pipe 33, an overflow pipe 34, and a solution supplying pipe35. In the solution tank 32, the measurement and the control of theconcentration of components in the electrolytic solution 20 are made. Inthe controller section 38 there are an acid concentration controller 39and a salt concentration controller 40, which are used for controllingthe concentration of the electrolytic solution 20 in the solution tank32, as described below. An upper end of the feed-back pipe 33 isconnected to a bottom of the electrolytic treatment bath 26 to form anopening through which the electrolytic solution 20 is fed back towardthe solution tank 32. In a middle of the feed back pipe 33, a valve 36is provided. When the electrolytic treatment is made, the valve 36 iscontrolled to open. Accordingly, the electrolytic solution 20 startsflowing through the feed-back pipe 33 to the solution tank 32 at anearly constant flow rate.

[0024] An upper end of the overflow pipe 34 is connected to a side wallof the electrolytic treatment bath 26, and a lower end is inserted intothe electrolytic solution 20 in the solution tank 32. When the surfaceof the electrolytic solution 20 in the electrolytic solution cell 26 ishigher than a lower edge of an opening of the overflow pipe 34 on theside wall of the electrolytic treatment bath 26, then the electrolyticsolution 20 starts flowing from the electrolytic solution cell 26through the overflow pipe 34 to the solution tank 32. Therefore, thelevel of the electrolytic solution 20 does not higher than thepredetermined upper limit level.

[0025] A lower end of the solution supplying pipe 35 is connected to alower side of a side wall of the solution tank 32, and an upper end ispositioned inside the electrolytic treatment bath 26 and above theelectrolytic solution 20. Further, the solution supplying pipe 35 isprovided with a pump 37. When the pump 37 is driven, the electrolyticsolution 20 is fed out from the solution tank 32 and supplied into theelectrolytic treatment bath 26. The electrolytic solution 20 iscircularly fed between the electrolytic treatment bath 26 and thesolution tank 32. Accordingly, when the concentration of components inthe electrolytic solution 20 is controlled in the solution tank 32, theconcentration of the electrolytic solution 20 in the electrolytictreatment bath 26 is kept in a predetermined range.

[0026] The control of the concentration of the electrolytic solution 20in the solution tank 32 is made in the controller section 38. Thecontroller section 38 controls the concentrations of the hydrochloricacid (as the main content) and the aluminum chloride in the electrolyticsolution 20 in respectively predetermined ranges. The aluminum chlorideis a metallic salt which is generated by the ionization of the aluminumatoms on the surface of the aluminum web 11 in the electrolytictreatment, and is dissolved to the electrolytic solution 20. When theconcentration of the aluminum chloride (hereinafter salt concentration)in the electrolytic solution 20 becomes lower, the reaction speed of thereaction in the electrolytic treatment becomes higher, and then the saltconcentration becomes higher, the reaction speed becomes lower. In thepresent invention, not only the hydrochloric acid but also theconcentration of the aluminum chloride are controlled such that theelectrolytic treatment to the transported metallic substrate isuniformly made. In this embodiment, the controller section 38 has theacid concentration controller 39 and the salt concentration controller40 so as to control the respective concentrations of the hydrochloricacid and the aluminum chloride in the electrolytic solution 20.

[0027] In order to control the concentration of the hydrochloric acid(hereinafter acid concentration) in the electrolytic solution 20, thehydrochloric acid is fed into the solution tank 32 in actuation of theacid concentration controller 39. In order to control the saltconcentration in the electrolytic solution 20, the water (preferably,pure water) is fed into the solution tank 32 in actuation of the saltconcentration controller 40 and added to the electrolytic solution 20 inthe solution tank 32. Further, each of the acid and salt concentrationcontrollers 39, 40 is a programmable controller which is constructed ofa CPU, a RAM and the like for operating input data.

[0028] As shown in FIG. 3, the acid concentration controller 39 isconnected to an acid concentration meter 41 and an acid feeding section42. The acid concentration meter 41 measures the acid concentration ofthe electrolytic solution 20 in the solution tank 32, for example onceat 10 seconds, and sends the acid concentration controller 39 an acidconcentration data obtained in the measurement. For example, as the acidconcentration meter 41, a densitometer, a titlator or the like is used.

[0029] An acid concentration data of the measured acid concentration ofhydrochloric acid is input into the acid concentration controller 39,and then sent to a difference discriminator 43. The differencediscriminator 43 calculates the difference the measured acidconcentration and the predetermined acid concentration of theelectrolytic solution 20, on the basis of the acid concentration data.When the difference is larger than a predetermined value, then thedifference discriminator 43 discriminates that it is necessary to addthe hydrochloric acid to the electrolytic solution 20 in the solutiontank 32. In this case, the acid concentration controller 39 sends anacid feed signal to the acid feeding section 42.

[0030] When the acid feed signal is input into the acid feeding section42, the acid feeding section 42 starts feeding the undilutedhydrochloric acid to the solution tank 32. The acid feeding section 42is constructed of an acid feed pipe 44, a hydrochloric acid tank (notshown), a solenoid valve 45, an automatic valve 46, a acid feed pump 47and the like. An end of the acid feed pipe 44 is connected to thehydrochloric acid tank, and another end is connected to the solutiontank 32 on the surface of the electrolytic solution 20. Further, theautomatic valve 46 and the acid feed pump 47 are positioned on the acidfeed pipe 44. The automatic valve 46 is opened and closed in accordancewith the operation of the solenoid valve 45. When receiving the acidfeed signal, the automatic valve 46 are opened. When receiving the acidfeed signal, the acid feed pump 47 is driven to feed the undilutedhydrochloric acid from the hydrochloric acid tank to the solution tank32. Thus the undiluted hydrochloric acid is added to the electrolyticsolution 20 in the solution tank 32.

[0031] As shown in FIG. 2, the salt concentration controller 40 isconnected to a salt concentration meter 48, the power source section 30,and a water feeding section 49. The salt concentration meter 48, as sameas the acid concentration meter 41, measures the salt concentration ofaluminum chloride in the electrolytic solution 20 contained in thesolution tank 32, and sent the salt concentration controller 40 a saltconcentration data of the measured salt concentration of aluminumchloride. For example, as the salt concentration meter 48, there are aAlCl₃ densitometer, a titlator or the like for calculating theconcentration of aluminum hydrochloride from electric conductivity orother physical quantities that are measured with use of ultrasonic waveand the like.

[0032] Further, the power source section 30 sends the salt concentrationcontroller 40 a current value data of a total current supplied duringthe electrolytic treatment. The total current value is total of currentvalues of the two electrodes since the currents are supplied to the twoelectrodes 28. A quantity of aluminum dissolving to the electrolyticsolution in a unit time varies in accordance with the current value ofthe total current. Note that the amount of metal (aluminum in thisembodiment) dissolved to the electrolytic solution in unit time isdetermined as electrolytic treatment quantity.

[0033] As shown in FIG. 3, the salt concentration data and a totalcurrent data input into the salt concentration controller 40 is sent toa water feed cycle operating unit 50 in the salt concentrationcontroller 40. The water feed cycle operating unit 50 calculates a cycleof supplying water (described below) on the basis of the input saltconcentration data and the input total current data. The saltconcentration controller 40 sends a water feed signal to the waterfeeding section 49 at every cycle calculated by the water feed cycleoperating unit 50. Further, the salt concentration controller 40 isprovided with a water feed period controller 51 which controls a periodfor feeding the predetermined amount of the water in each water feedperiod.

[0034] The water feeding section 49 feeds the water into the solutiontank 32 to add to the electrolytic solution 20 when the water feedsignal is input. The water feeding section 49 substantially has the samestructure as the acid feeding section 42, and constructed of an waterfeed pipe 52, a water tank (not shown), a solenoid valve 53, anautomatic valve 54, a water feed pump 55 and the like.

[0035] The acid concentration data generated in the acid concentrationmeter 41 is input through an AI (Analog Input) into the acidconcentration controller 39. A measured value PV_(b) of the acidconcentration of the hydrochloric acid is displayed on a panel (notshown) provided for the controller section 38 or a display of a personalcomputer (not shown) connected to the controller section 38.

[0036] A correction value is added to the measured value PV_(b) in orderto correct the error of the measuring device or the measuring method.After the correction, the corrected measured value PV_(b) is displayedon a display panel. In this case, when the measured value PV_(b) is notin the predetermined range, a warning notice is displayed.Simultaneously, the measured value PV_(b) after the correction is sentto the difference discriminator 43, so as to perform an operation forand discrimination of controlling the acid concentration of thehydrochloric acid. And a setting value SV_(b) of acid concentration anda preset value e are input through a keyboard into the differencediscriminator 43. When the difference from the measured value PV_(b) tothe setting value SV_(b) is larger than the preset value e, namelySV_(b)−PV_(b)>e, the acid feed signal is generated and sent through a DO(DigitalOut) to the solenoid valve 45 and the acid feed pump 47.Thereby, an image representing the feed of the hydrochloric acid isdisplayed.

[0037] The salt concentration data sent from the salt concentrationmeter 48 is input into the salt concentration controller 40. Thecorrection value is added to the measured value PV_(a) of the saltconcentration of the aluminum chloride, and then the measured valuePV_(a) is sent to the water feed cycle operating unit 50. Further, atotal current value I of the current supplied during the electrolytictreatment and the preset value SV_(a) of the salt concentration areinput.

[0038] In the water feed cycle operating unit 50, the inverse of thecurrent value I is multiplied by an optional coefficient A, and furtheradded to an optional coefficient B. Thus the standard feed water cycleT_(o) is obtained in the following formula:

T _(o) =A/I+B

[0039] The standard feed water cycle T_(o) is determined as a cycle forwater supplying depending on electrolytic treatment quantity. In theactual electrolytic treatment apparatus, as there are many elements oferrors, the correction must be made to the standard feed water cycleT_(o). Accordingly, the water feed cycle operating unit 50 calculate afeed water cycle in the following formula wherein C, D are opticalcoefficients:

T=T _(o)×(1+C×(PV _(a) −SV _(a)))+D

[0040] The optional coefficients A-D varies according to material andtransporting speed of the aluminum web 11, concentration and temperatureof the electrolytic solution 20, largeness and number of the electrodes,and the like. Accordingly, it is necessary to set a predetermined valueaccording to the variation of the aluminum material and the electrolytictreatment apparatus. On the basis of the calculated feed water cycle T,the water feed cycle operating unit 50 sends the water feed signalthrough the water feed period controller 51 to the solenoid valve 53 andthe water feed pump 55. Note that since the feed water period forfeeding water is previously input into the water feed period controller51, the water feed signal is generated for the feed water period forfeeding water. Further, an image showing the feeding water is displayed.

[0041] As described above, as a simple operation formula is used forcontrolling the concentrations of hydrochloric acid and aluminumhydrochloride in the electrolytic solution 20. Accordingly, the controlof the concentration can be made with use of a cheap programmablecontroller even though the expensive control device, such as PIDcontroller, is not used.

[0042] Effects of the above embodiment will be explained, and theexplanation start with the control process of the acid concentration ofthe hydrochloric acid. The measured value PV_(b) of the acidconcentration in the electrolytic solution 20 is measured by the acidconcentration meter 41, and sent to the difference discriminator 43 inwhich the setting value SV_(b) of the acid concentration and the presetvalue e are previously input.

[0043] In the condition of SV_(b)−PV_(b)>e, the difference discriminator43 outputs the acid feed signal to the solenoid valve 45 and the acidfeed pump 47. And the drive of the acid feed pump 47 starts, and thehydrochloric acid is fed to the electrolytic solution 20 in the solutiontank 32. Thus the acid concentration of the electrolytic solution 20becomes higher. When the measured value PV_(b) of the acid concentrationsatisfies the condition of SV_(b)−PV_(b≦e), the difference discriminator43 stops sending the acid feed signal. Corresponding to the stop of theacid feed signal, the solenoid valve 45 is operated to close theautomatic valve 46, and the drive of the acid feed pump 47 stops. Theacid concentration meter 41 continuously measures the acidconcentration, and in the condition SV_(b)−PV_(b)>e, the acidconcentration of the hydrochloric acid in the electrolytic solution 20is controlled by repeating the above processes.

[0044] Then, the processes of controlling the salt concentration of thealuminum chloride will be explained. The data of the total current valueI of the current supplied during the electrolytic treatment and that ofthe measured value PV_(a) of the salt concentration which is measured bythe salt concentration meter 48 are sent to the water feed cycleoperating unit 50 in which the preset value SV_(a) is input. Then thefeed water cycle T is calculated.

[0045] At every feed water cycle T, the water feed signal is output tothe solenoid valve 53 and the water feed pump 55. When the water feedsignal is output, the solenoid valve 53 opens the automatic valve 54.Simultaneously, the drive of the water feed pump 55 starts to feed thewater in the water tank to the solution tank 32 to add to theelectrolytic solution 20. The feed water period is controlled by thewater feed period controller 51. when the feed water period passes, thesolenoid valve 53 closes the automatic valve 54, and the feed water pumpstops. The salt concentration in the electrolytic solution 20 iscontrolled by repeating the above processes at every feed water cycle T.

[0046] In this embodiment of the electrolytic treatment apparatus forthe production line of PS plate, an aluminum material is dipped into theelectrolytic solution whose main content is hydrochloric acid, and theelectrolytic treatment of the aluminum material is made. However, thepresent invention can be applied to treatment apparatus of a surface ofseveral metallic materials with electrolytic reaction in theelectrolytic solution, for example, to a oxidization apparatus for theproduction line of PS plate, other several treatment apparatus, and thelike.

[0047] Various changes and modifications are possible in the presentinvention and may be understood to be within the present invention.

What is claimed is:
 1. A method for controlling a concentration of anelectrolytic solution for making an electrolytic treatment of a metallicmaterial, comprising steps of: measuring a acid concentration of acid insaid electrolytic solution; measuring a salt concentration of salt whichis generated by ionizing part of said metallic material in saidelectrolytic solution in said electrolytic treatment; and adding atleast one of a diluting liquid and a fresh acid according to saidmeasured acid concentration, said measured salt concentration, and acurrent value of said electrolytic current supplied during saidelectrolytic treatment.
 2. A method as claimed in claim 1, furthercomprising a step of calculating a feed cycle of adding a predeterminedamount of said diluting liquid from said measured salt concentration andsaid current value.
 3. A method as claimed in claim 2, furthercomprising steps of: calculating a difference from said measured acidconcentration to an objected acid concentration; and adding said freshacid to said electrolytic solution when said difference is larger than apredetermined limit value.
 4. A method as claimed in claim 2, whereinwhen said current value is I, and A and B are optional constants, astandard cycle T_(o) for adding said diluting liquid to saidelectrolytic solution is T_(o)=A/I+B, and wherein when said measuredsalt concentration is PV_(a), said objected salt concentration isSV_(a), and C and D are optional constants, said feed cycle T for addingthe predetermined amount of said diluting liquid is, T=T _(o)×(1+C×(PV_(a) −SV _(a)))+D.
 5. A method as claimed in claim 4, wherein saidmetallic material is an aluminum plate used for a substrate of a PSplate.
 6. A method as claimed in claim 5, wherein said acid ishydrochloric acid.
 7. An apparatus for controlling a concentration of anelectrolytic solution in which an electrolytic treatment of a metallicmaterial is made, comprising: an acid concentration meter for measuringan acid concentration of acid contained in said electrolytic solution; asalt concentration meter for measuring a salt concentration of saltwhich is generated by ionizing part of said metallic material in saidelectrolytic treatment; a current meter for measuring a current value ofsaid electrolytic current supplied during said electrolytic treatment;an acid concentration controller for controlling said acidconcentration, said acid concentration controller adding a fresh acid tosaid electrolytic solution on the basis of a measured acid concentrationand an objected acid concentration; and a salt concentration controllerfor controlling said salt concentration of said metallic salt, said saltconcentration controller calculating a controlling period of said saltconcentration on the basis of a measured salt concentration and saidcurrent value, to add a diluting liquid to said electrolytic solution ateach controlling period.
 8. An apparatus as claimed in claim 7, whereinwhen said current value is I, and A and B are optional constants, astandard cycle T_(o) for adding said diluting liquid to saidelectrolytic solution is T_(o)=A/I+B, and wherein when said measuredsalt concentration is PV_(a), said objected salt concentration isSV_(a), and C and D are optional constants, said feed cycle T forfeeding said diluting liquid is, T=T _(o)×(1+C×(PV _(a) −SV _(a)))+D.